My iMac has been shutting down every time during sleep. I have tried removing all peripherals, and replacing the USB hub, in case that was bad. Still happening.
Once the machine shuts down during sleep, it will not restart without the SMC trick. It's pretty scary when you have to unplug a Mac, and replug it back in to get it to even turn on!! I've also done the PRAM reset a few times. Though this still happens.
The last time this happened, I had the usb hub plugged in, which has a small light on it. The light was on during the Mac's sleep, but when I pressed one of the keys on the keyboard to wake the machine, the light went out. So, I suspect that the Mac is actually shutting down on the "wake" part.
My mouse and keyboard are both wireless. Any chance this is a wifi issue? Perhaps I would try a wired mouse or keyboard?
My Mac is less than 3 years old, so still under Apple Care. Will try them if no answers found here. 🙂